1. Field of the Invention
The present invention relates to a process for the surface treatment of materials, in particular of stony materials, marble, sandstone, bricks, concretes and the like, and of metals, and to aqueous compositions employed therein.
2. Description of the Related Art
As is known, stony materials, marble, sandstone, bricks and concretes, usually employed in building industry, and also metals, suffer from a progressive decay owing to the atmospheric agents and pollutants. Such decay is determined by a series of phenomena both of physical and chemical nature which mainly occur in the presence of water. In order to eliminate or at least meaningfully reduce, such phenomena, it is known to apply on the surface of such materials protective substances of various kinds (see "Stone Decay and Conservation", of Amoroso e Fassina, Elsewier Ed., Amsterdam, 1983).
If these decay phenomena have induced the formation of a surface layer having poor cohesion, it is essential that the applied substances, besides acting as protective agents, also exert a surface aggregating action.
Briefly, a good aggregating/protective agent, to perform the desired action and not to contemporanteously induce side decay phenomena or other undesired secondary effects, must show the following characteristics:
1. to markedly reduce the inlet and the penetration rate of water inside the material to be protected and consolidated; PA0 2. to re-establish the cohesion between the particles of the surface layer of the decayed material, as well as the adhesion between the surface layer and the integral underlying part; PA0 3. to impart an oil-repellence effect so as to reduce the penetration of oils and hydrocarbons, commonly present as atmospheric pollutants, or at least to make their removal from the material easier; PA0 4. to show suitable chemical stability with respect to the action of the atmospheric agents, of light and of heat; PA0 5. to maintain elasticity properties in order to avoid the formation of a stiff surface layer with mechanical properties different with respect to the subtratum; PA0 6. to show chemical inertia towards the material to be treated; PA0 7. not to alter, where required, the chromaticity of the material to be treated; PA0 8. to maintain over time its own characteristics of solubility to assure the treatment reversibility; PA0 9. to maintain a sufficient permeability to air and to steam, in order to avoid, because of the condensate, alteration phenomena under the protected surface layer. PA0 (a) from 2 to 95% by weight, preferably from 2 to 60%, even more preferably from 2 to 30% by weight based on the weight of the dry product of an inorganic compound of the silicon selected from: alkaline, alkaline-earth metals, ammonium silicates, colloidal silica, or their mixtures; and PA0 (b) from 5 to 98%, preferably from 40 to 98%, even more preferably from 70 to 98% by weight based on the weight of dry product of a fluoroelastomer based on vinylidene fluoride (VDF).
In the class of fluorinated polymers, the use of perfluoropolyoxyalkylenes as protective agents for stony and cement manufactured articles, as described in U.S. Pat. No. 4,499,146 and U.S. Pat. No. 4,745,009, is known. Such products, even though they solve the problem of the protection, cannot however exert an effective consolidating action because of their nature of fluids.
A substantial improvement has been obtained by using elastomeric polymers based on vinylidene fluoride (VDF), which effectively exert both a protective and aggregating function (see for instance U.S. Pat. No. 4,764,431). A further improvement has been obtained by using as aggregating/protective agents, mixtures consisting of a fluoroelastomer based on VDF and of a VDF non elastomeric homopolymer or copolymer, as described in EP patent EP-A-479,240.
The use of silica and inorganic silicates as reinforcing agents for both non fluorinated and fluorinated resins is also known. For instance, in U.S. Pat. No. 4,680,331 aqueous compositions are described, consisting of a fluorinated resin, in particular polytetrafluoroethylene (PTFE), a silicone emulsion, a flocky inorganic material and a non ionic surfactant, optionally in admixture with an anionic surfactant. Among the inorganic flocky materials SiO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3 and others are indicated. Such compositions are employed for the coating of glass or metal. To obtain a stable coating, the fluorinated resin must be submitted after the application to a sintering process at a temperature higher than the melting point of the resin itself, for example, higher than 320.degree. C. in case of PTFE. The use of so high temperatures is a true drawback, both as it makes the process applicable only on manufactured thermally resistant articles of small sizes, and because of unavoidable decay phenomena of the silicone material.
The Applicant has now surprisingly found that the application of an aqueous composition, as defined hereinunder, comprising both a fluoreoelastomer and an inorganic compound of the silicon, allows one to obtain a remarkable improvement in surface protection and consolidation compared to the individual components when used separately. The synergistic effect of the combination of an inorganic compound of silicon and a fluoroelastomer is particularly surprising since, on the basis of experiments carried out by the Applicant, the application of an inorganic compound of silicon, for example sodium silicate, on a stony material while exerting a discrete consolidating action, does not produce any protective effect, since the silicate does not possess water-repellent properties.